It is known from U.S. Pat. No. 5,157,597 how to detect oscillations in a servo system during operation and to change the loop amplification of control loops. In the process, a characteristic curve is established for the ratio of the amplification of the position control loop with respect to the position deviation in such a way that the loop amplification for defined values of the position deviation assumes a large value. During operation, the position deviation is periodically determined and the main oscillation component of the position deviation is determined. Finally, the loop amplification of the servo system is adjusted in such a way that the main oscillation component falls into a defined frequency range.
A disadvantage, however, lies in that during the running time, the frequency range in which the servo system starts to oscillate is adjusted, but the oscillation tendency is not reduced. Therefore no optimization of the controller parameters takes place. The set frequency range can be displaced in different load stages which can lead to oscillation and therefore to damage of the servo system.
A method for adjusting a PID controller is known from U.S. Pat. No. 4,549,123. In this case a non-linear component whose output signal has either a constant negative or positive amplitude, is connected upstream of the PID controller. Thereafter the controller parameters are changed in such a way that the transfer function of the non-linear component, multiplied by the mutual transfer function of the controller and the control system for a defined amplitude and a defined frequency, results in the value of -1 and the control circuit changed in this way can start to oscillate at this point. Thereafter different core values are calculated in accordance with the equations of Ziegler and Nichols, and the controller is set as a function of the calculated core values.
In this case it is disadvantageous that the entire control structure must be changed, it is not provided to only perform a change of the controller parameters.
It is known from European Patent Publication EP 0 347 465 B1 that a servo motor has a greater tendency to oscillate at a low rpm than at a high rpm. Therefore a control circuit is provided for the servo motor whose amplification in the P and I branches of the controller is set as a function of the rpm. By means of this it is achieved that the servo motor does not start to oscillate at a low rpm, and no unnecessary limitation of the torque takes place at a high rpm.
In this case it is disadvantageous that optimal starting values for a stable operation must already be provided, which thereafter are merely matched to a change in rpm of the electric motor. There is no disclosure of how the starting values are determined.
As a whole, the methods for suppressing oscillations in controlled electric motors which can be found in the prior art, do not take into consideration that an electric motor has been inserted into a mechanical arrangement which can considerably change the oscillating tendency of the entire drive system. Based on additional mechanical components for transferring the movement generated by the electric motor which, when they are employed, are permanently connected with the motor, the determination of the oscillating tendency of only the electric motor is not sufficient.